Self-repairing metal alloy matrix composites, methods of manufacture and use thereof and articles comprising the same

ABSTRACT

Disclosed herein is a composite comprising a metal alloy matrix; where the metal alloy matrix comprises aluminum in an amount greater than 50 atomic percent; a first metal and a second metal; where the first metal is different from the second metal; and where the metal alloy matrix comprises a low temperature melting phase and a high temperature melting phase; where the low temperature melting phase melts at a temperature that is lower than the high temperature melting phase; and a contracting constituent; where the contracting constituent exerts a compressive force on the metal alloy matrix at a temperature between a melting point of the low temperature melting phase and a melting point of the high temperature melting phase or below the melting points of the high and low temperature melting phases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims priority to U.S. Provisional Application No.62/011222 filed Jun. 12, 2014, the entire contents of which are herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support under Contract NumberNNX12AP71A awarded by NASA. The government has certain rights in theinvention.

BACKGROUND

This disclosure relates to self-repairing metal alloy matrix composites,methods of manufacture and use thereof and articles comprising the same.

Metal materials, such as aluminum alloys, have been investigated for usein aerospace equipment. Although properties of aluminum such as cost andweight are desirable for aerospace and other lightweight materialapplications, during operation such materials can be subjected tophysical stresses due to cyclic loading, resulting in the formation offatigue cracks, and subsequently, may lead to fatigue failure.

To repair fatigue cracks, additional materials, skilled application of arepair technique and/or direct access are generally utilized. Forexample, repair methods may include drilling the crack tip to blunt thecrack in order to prevent further crack propagation, grinding out thecrack and using weld overlays to obtain the desired part thickness,using a doubler to reinforce the material surrounding the crack,applying selective plating over the crack, applying polymeric fillers tofill the crack, or applying thermal sprays over the crack. However,since these repair techniques involve additional materials, skilledexternal application and/or direct access, these techniques may not bedesirable in aerospace applications, such as those in space flight.Further, repair techniques involving doublers pose challenges withregard to bonding and surface preparation in aerospace application. Inaddition, weld overlays, while suitable for some metal alloys such assteel-based alloys, pose challenges in terms of reduced strength whenapplied to aluminum-based structural materials.

It is therefore desirable to develop metal alloy matrix composites andmethods of making thereof which have self-repairing capabilities tofacilitate the closure and/or repair of fatigue cracks and/or to avoidone or more of the challenges described above.

SUMMARY

Disclosed herein is a composite comprising a metal alloy matrix; wherethe metal alloy matrix comprises aluminum in an amount greater than 50atomic percent; a first metal and a second metal; where the first metalis different from the second metal; and where the metal alloy matrixcomprises a low temperature melting phase and a high temperature meltingphase; where the low temperature melting phase melts at a temperaturethat is lower than the high temperature melting phase; and a contractingconstituent; where the contracting constituent exerts a compressiveforce on the metal alloy matrix at a temperature between a melting pointof the low temperature melting phase and a melting point of the hightemperature melting phase or below the melting point of the low and highmelting phase.

Disclosed herein too is a method comprising blending aluminum, a firstmetal, a second metal and a contracting constituent to form a composite;where the first metal is different from the second metal; and where thealuminum, the first metal and the second metal form a metal alloy matrixthat comprises a low temperature melting phase and a high temperaturemelting phase; where the low temperature melting phase melts at atemperature that is lower than the high temperature melting phase; wherethe contracting constituent exerts a compressive force on the metalalloy matrix at a temperature between a melting point of the lowtemperature melting phase and a melting point of the high temperaturemelting phase or below the melting point of the low and high meltingphase; and forming the composite into an article.

Disclosed herein too is a method comprising heating a compositecomprising a metal alloy matrix; where the metal alloy matrix comprisesaluminum in an amount greater than 50 atomic percent; a first metal anda second metal; where the first metal is different from the secondmetal; and where the metal alloy matrix comprises a low temperaturemelting phase and a high temperature melting phase; where the lowtemperature melting phase melts at a temperature that is lower than thehigh temperature melting phase; and a contracting constituent; where thecontracting constituent exerts a compressive force on the metal alloymatrix at a temperature between a melting point of the low temperaturemelting phase and a melting point of the high temperature melting phaseor below the melting point of the low and high melting phase; andcooling the composite.

Disclosed herein is a composite comprising a metal alloy matrix; wherethe metal alloy matrix comprises aluminum in an amount greater than 50to 99.9 atomic percent; a first metal dispersed in the metal alloymatrix, where the first metal is present in an amount of 0.1 to 50atomic percent; and where the metal alloy matrix comprises a lowtemperature melting phase and a high temperature melting phase; wherethe low temperature melting phase melts at a temperature that is lowerthan the high temperature melting phase; and a contracting constituent;where the contracting constituent exerts a compressive force on themetal alloy matrix at a temperature between a melting point of the lowtemperature melting phase and a melting point of the high temperaturemelting phase or below the melting point of the low and high meltingphase;.

Disclosed herein too is a method comprising blending aluminum a firstmetal, and a contracting constituent to form a composite; and where thealuminum and the first metal form a metal alloy matrix that comprises alow temperature melting phase and a high temperature melting phase;where the low temperature melting phase melts at a temperature that islower than the high temperature melting phase; where the contractingconstituent exerts a compressive force on the metal alloy matrix at atemperature between a melting point of the low temperature melting phaseand a melting point of the high temperature melting phase or below themelting point of the low and high melting phase; and forming thecomposite into an article.

Disclosed herein too is a method comprising heating a compositecomprising a metal alloy matrix; where the metal alloy matrix comprisesaluminum in an amount greater than 50 to 99 atomic percent; a firstmetal dispersed in the metal alloy matrix, where the first metal ispresent in an amount of 1 to 50 atomic percent; and where the metalalloy matrix comprises a low temperature melting phase and a hightemperature melting phase; where the low temperature melting phase meltsat a temperature that is lower than the high temperature melting phase;and

a contracting constituent; where the contracting constituent exerts acompressive force on the metal alloy matrix at a temperature between amelting point of the low temperature melting phase and a melting pointof the high temperature melting phase or below the melting point of thelow and high melting phase; and cooling the composite.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(A) is a depiction of the composite comprising a metal alloymatrix and a contracting constituent;

FIG. 1(B) is a depiction of the composite with a crack;

FIG. 1(C) is a depiction of the composite after it is heated to abovethe melting point of the lower melting temperature phase and below thetemperature of the higher melting temperature phase or below the meltingpoint of the low and high melting phase;

FIG. 1(D) is a depiction of section X-X′ of FIG. 1(B) showing the moltenportion of the low temperature melting phase when it is heated to aboveits melting point;

FIG. 2 shows phase diagram of Al—Si below 50% Si. At a composition ofAl-3.0 at % Si (3.1 wt % Si), a healing temperature of 592° C. willyield 20% liquid;

FIG. 3 shows the phase fraction of Al-3.0 at % Si at varioustemperatures. The heat-treatment temperature of 592° C. reveals a 20%liquid composition during the healing process. The box signifies thepotential area of liquid % should the temperature fluctuate by ±5° C.during heat-treatment;

FIG. 4 shows representative microstructure of an Al-3.0 at % Si (3.1 wt% Si) composite after heat-treatment at 592° C. for 24 hours;

FIG. 5 depicts representative microstructure in an Al-3.0 at % Si (3.1wt % Si) composite after heat-treatment at 592° C. for 24 hours showingfailure occurring along eutectic regions;

FIG. 6 shows a comparison of the a) pre-heal to b) post-heal tensile barin an Al-3.0 at % Si composite reinforced with 2.0 vol % NiTi SMA wire;

FIG. 7 shows a comparison of the virgin and healed tensile behavior ofan Al-3.0 at % Si composite reinforced with 2.0 vol % NiTi SMA wire. Thesample was found to possess 90.6% retained strength post-healingheat-treatment;

FIG. 8 shows a comparison of the a) post-tensile 1, b) post-heal and c)post-tensile 2 in an Al-3.0 at % Si composite reinforced with 2.4 vol %NiTi SMA wire;

FIG. 9 shows a comparison of the virgin tensile behavior of an Al-3.0 at% Si composite reinforced with either 2.26 vol % or 4.43 vol % NiTi SMAwire;

FIG. 10 shows an optical image showing debonding between an Al-3 at % Simatrix and a NiTi wire following tensile testing.

FIG. 11 depicts a comparison of S-N data from Al-3Si and A201,normalized with respect to UTS;

FIG. 12 shows optical micrographs of cracks extending from apre-machined notch in M(T) sample;

FIG. 13 shows fatigue crack growth rate from M(T) testing of Al—Si SMASHalloy. All cracks showed some crack closure, as well as reduction incrack growth rate. The grey area represents the location of SMAreinforcement;

FIG. 14 shows how the crack propagated after healing. While the crackinitially propagates intergranularly following the eutectic along thegrain boundaries, it changes to transgranular propagation once the crackextended beyond previously healed sections of the matrix material. Thefigure shows an optical micrograph of AlSi ESE(T) sample after secondaryfatigue cracking post initial healing; and

FIG. 15 below shows a graph of crack length versus the number of cyclesfor all three fatigue tests. The figure shows ESE(T) crack growth rateof Al—Si matrix after two healing treatment cycles. It can be seen thatafter the 1^(st) healing, the crack length returns back to the original,pre-healed pre-crack length (indicating that the healing cycle causedsignificant crack closure). After the second healing cycle, although thecrack length did not return back to its original size, it did recoversome of the damage (closing 2 mm of crack length) from the secondfracture/healing event.

DETAILED DESCRIPTION Aluminum-Silicon System

Disclosed herein is a self-healing composite (hereinafter the“composite”) that comprises a metal alloy matrix in which is reinforcedby a constituent that undergoes contraction upon heating (hereinafterthe “contracting constituent”). The metal alloy matrix comprises a metalalloy having a plurality of phases that contain the same elements atleast one of which is a high temperature melting phase while the othersare a low temperature melting phase. In other words, the one or more lowtemperature melting phases melt at a lower temperature than the hightemperature melting phase.

The composite is capable of self-repairing large-scale cracks that havepropagated through the matrix during the service life of the component.When a crack is present in the metal alloy matrix, the composite isheated in order to facilitate self-healing. The heating is conducted toa temperature where the lower temperature melting phase undergoesmelting and where the contracting constituent is activated to begincontracting. The compressive forces brought about by the contractingconstituent draws the opposing surfaces (on either surface of the crack)together where the low temperature melting phase (now in the form of aliquid) facilitates bonding of the two opposing surfaces. The compositeis thus repaired without any significant use of external compressiveforces, without using any techniques that are used for repair such aswelding, brazing, soldering, bonding with an adhesive, or the like.

FIG. 1(A) is a depiction of an article 100 that comprises the exemplaryself-healing metal matrix alloy 102 in which is dispersed thecontracting constituent 104. As the article is subjected to externalloading as seen in the FIG. 1(B), a crack 106 develops and propagatesthrough the metal matrix alloy 102 but not through the contractingconstituent 104. Upon heating the composite article 100 (as seen in theFIG. 1(C)) to the desired temperature, the contracting constituentundergoes a phase transformation bringing the opposing crack facestogether, while at the same time, the melting phase undergoes meltingand facilitates the bonding of the opposing crack faces by filling inany remaining gaps , thus repairing the article 100. This is a two-stepcrack repair method, first is crack closure from contraction of thecontracting constituent, and second is crack repair during partialliquefaction of the matrix. Both steps are accomplished by heating thecrack area to a predetermined temperature.

The FIG. 1(D) is an expanded depiction of the section X-X′ of the FIG.1(B). As can be seen in the FIG. 1(D), when the composite is heated to atemperature above the melting point of the low temperature meltingphase, the low temperature melting phase melts to form a liquid 102A.The liquid 102A spreads along the interface. When the contractingconstituent exerts a compressive force on the opposing surfaces of thecrack, the liquid at the interface spreads along the interface andsolidifies upon cooling, thus promoting self-healing of the composite.The liquid 102A may form a film along the entire crack or along only aportion of the crack.

The metal matrix alloy comprises a base metal (i.e., a metal that ispresent in an amount large enough to act as the matrix of the metalmatrix alloy) that may be selected from the group consisting ofaluminum, copper, tin, lead, cadmium, zinc, indium, bismuth, gallium,magnesium, lithium, calcium, silicon, antimony, or the like. In oneembodiment, the metal matrix alloy comprises an aluminum base.

To the base metal is added a first metal. The first metal is capable ofmixing with the aluminum to produce two or more phases having differentmelting temperatures. One of these phases is a high temperature meltingphase and the other phase is a low temperature melting phase. The basemetal is always different from the first metal. In an exemplaryembodiment, the first metal is silicon, copper, tin, lead, cadmium,zinc, indium, bismuth, silicon, gallium, magnesium, lithium, calcium,antimony, or the like. In an exemplary embodiment, the first metal issilicon.

The aluminum is present in an amount of 50 to 99.9, specifically 60 to98, and more specifically 70 to 97 atomic percent, based on the totalatomic composition of the composite. The silicon is present in an amountof 1 to 50, specifically 2 to 40, and more specifically 3 to 30 atomicpercent, based on the total atomic composition of the composite. In anexemplary embodiment, the nominal bulk composition of the metal alloymatrix is: Al-3.0 Si (all compositions in atomic percent).

The aluminum silicon alloy has a high temperature melting phase and alow temperature melting phase. The low temperature melting phase isgenerally present in an amount of 5 to 23 volume percent specifically 18to 22 volume percent, based on the volume of the metal.

The lower temperature melting phase generally melts at a temperature of560 to 620° C., specifically 580 to 610° C. and more specifically 585 to600° C.

Aluminum-Silicon-Copper System

Disclosed herein is a self-healing composite (hereinafter the“composite”) that comprises a metal alloy matrix in which is reinforcedby a constituent that undergoes contraction upon heating (hereinafterthe “contracting constituent”). The metal alloy matrix comprises a metalalloy having a plurality of phases that contain the same elements atleast one of which is a high temperature melting phase while the othersare a low temperature melting phase. In other words, the one or more lowtemperature melting phases melt at a lower temperature than the hightemperature melting phase.

The composite is capable of self-repairing large-scale cracks that havepropagated through the matrix during the service life of the component.When a crack is present in the metal alloy matrix, the composite isheated in order to facilitate self-healing. The heating is conducted toa temperature where the lower temperature melting phase undergoesmelting and where the contracting constituent is activated to begincontracting. The compressive forces brought about by the contractingconstituent draws the opposing surfaces (on either surface of the crack)together where the low temperature melting phase (now in the form of aliquid) facilitates bonding of the two opposing surfaces. The compositeis thus repaired without any significant use of compressive forces,without using any techniques that are used for repair such as welding,brazing, soldering, bonding with an adhesive, or the like.

FIG. 1(A) is a depiction of an article 100 that comprises the exemplaryself-healing metal matrix alloy 102 in which is dispersed thecontracting constituent 104. As the article is subjected to externalloading as seen in the FIG. 1(B), a crack 106 develops and propagatesthrough the metal matrix alloy 102 but not through the contractingconstituent 104. Upon heating the composite article 100 (as seen in theFIG. 1(C)) to the desired temperature, the contracting constituentundergoes a phase transformation bringing the opposing crack facestogether, while at the same time, the melting phase undergoes meltingand facilitates the bonding of the opposing crack faces by filling inany remaining gaps, thus repairing the article 100. In one embodiment,the contracting constituent first bring about crack closure, and thenthe liquefaction of the lower melting phase occurs thus facilitatinghealing of the crack. This is a two-step crack repair method, first iscrack closure from contraction of the contracting constituent, andsecond is crack repair during partial liquefaction of the matrix. Bothsteps are accomplished by heating the crack area to a predeterminedtemperature.

The FIG. 1(D) is an expanded depiction of the section X-X′ of the FIG.1(B). As can be seen in the FIG. 1(D), when the composite is heated to atemperature above the melting point of the low temperature meltingphase, the low temperature melting phase melts to form a liquid 102A.The liquid 102A spreads along the interface. When the contractingconstituent exerts a compressive force on the opposing surfaces of thecrack, the liquid at the interface spreads along the interface andsolidifies upon cooling, thus promoting self-healing of the composite.The liquid 102A may form a film along the entire crack or along only aportion of the crack.

The metal matrix alloy comprises a base metal (i.e., a metal that ispresent in an amount large enough to act as the matrix of the metalmatrix alloy) that may be selected from the group consisting ofaluminum, copper, tin, lead, cadmium, zinc, indium, bismuth, gallium,magnesium, lithium, calcium, silicon, antimony, or the like. In oneembodiment, the metal matrix alloy comprises an aluminum base.

To the base metal is added a first metal and a second metal. The firstmetal and the second metal are capable of mixing with the aluminum toproduce two or more phases having different melting temperatures. Thebase metal is always different from the first metal and the secondmetal. The first metal is always different from the second metal. In anexemplary embodiment, the first metal is aluminum, copper, tin, lead,cadmium, zinc, indium, bismuth, gallium, magnesium, lithium, calcium,antimony, or the like, while the second metal is silicon. In anotherexemplary embodiment, the first metal is copper, while the second metalis silicon.

The aluminum is present in an amount of 65 to 96, specifically 80 to 94,and more specifically 85 to 93 atomic percent, based on the total atomiccomposition of the composite. The copper is present in an amount of 2 to18, specifically 3 to 17, and more specifically 4 to 16 atomic percent,based on the total atomic composition of the composite. The silicon ispresent in an amount of 1 to 17, specifically 2 to 16, and morespecifically 3 to 15 atomic percent, based on the total atomiccomposition of the composite. In an exemplary embodiment, the nominalbulk composition of the metal alloy matrix is: Al-4.1 Cu-2 Si (allcompositions in atomic percent).

As noted above, the metal alloy matrix comprises a high temperaturemelting phase (also called the solid solution phase) and a lowtemperature melting (also referred to as the eutectic or liquid phase).In an exemplary embodiment, the composition at lower temperatures (attemperatures that are below the healing temperature range) for the lowertemperature melting phase (i.e., the eutectic composition) is Al-15Cu-7.3 Si (all compositions in atomic percent).

In one embodiment, the lower temperature melting phase may containcopper in an amount of 10 to 18 atomic percent and silicon in an amountof 5 to 10 atomic percent, based upon the total atomic content of thecomposition.

The higher temperature melting phase at temperatures that are below thehealing temperature range (i.e., the solid solution phase) has acomposition of Al-(0.5 to 2) Cu-(0.25 to 1) Si (all compositions inatomic percent). In one embodiment, the higher temperature melting phasecomprises 0.1 to 4 atomic percent copper and 0.1 to 2 atomic percentsilicon, based upon the total atomic content of the composition.

In another embodiment, at higher temperatures (i.e., at temperaturesthat are in the healing temperature range), the higher temperaturemelting phase has a composition of Al-1.1 Cu-0.6 Si (all compositions inatomic percent), while the lower temperature melting phase has acomposition of): Al-13.4 Cu-6.6 Si (all compositions in atomic percent).

In one embodiment, the lower temperature melting phase undergoes meltingat a temperature that is at least 50° C., specifically at least 100° C.and more specifically at least 150° C. lower than the temperature thatthe higher temperature melting phase undergoes melting.

The Contracting Constituent

In an exemplary embodiment, the contracting constituent comprises ashape memory alloy that is dispersed through the composite in the formof wires, fibers, whiskers, pillars, rods, or the like. The contractingconstituent may be uniformly dispersed (i.e., the composite isisotropic), or alternatively, the contracting constituent may beheterogeneously dispersed (anisotropic) as may be seen in the FIGS.1(A)-1(C). As can be seen in the FIGS. 1(A)-1(C), the contractingconstituent is unidirectional. In an exemplary embodiment, thecontracting constituent may be in the form of nanorods, nanowires,nanotubes, microwires, microtubes, or microrods.

The prefix “nano” as used herein refers to materials having diameters ofless than 100 nanometers, while the prefix “micro” refers to materialshaving diameters of greater than 100 nanometers to 1000 micrometers.

Examples of the contracting constituent are shape memory materials ormaterials that have a negative coefficient of thermal expansion. It isdesirable for the contracting constituent to apply a compressive forceto the opposing surfaces of the crack. The contraction can occurimmediate after composite failure or when heated to the appropriatetemperature. In shape memory materials, this usually implies that thematerial undergoes reversion to its original state. The shape memorymaterial can be any shape memory material so long as the shape memorymaterial exhibits a shape memory effect upon heating to the temperatureat which the lower temperature melting phase undergoes melting. In apreferred embodiment, the shape memory material is a shape memory alloy.

Suitable shape memory alloy materials are nickel-titanium based alloys(including high temperature modifications such as Ti(NiPt), Ti(NiPd),Ti(NiAu), (TiHf)Ni, NiTiSn, and the like), indium-titanium based alloys,nickel-aluminum based alloys, nickel-gallium based alloys, copper basedalloys (e.g., copper-zinc-aluminum alloys, copper-aluminum-nickelalloys, copper-gold, and copper-tin alloys), gold-cadmium based alloys,silver-cadmium based alloys, indium-cadmium based alloys,manganese-copper based alloys, iron based alloys, (e.g., iron-platinumbased alloys, iron-palladium based alloys, iron-manganese alloys, andiron-chromium alloys) and the like. The alloys can be binary, ternary,or any higher order so long as the alloy composition exhibits a shapememory effect, e.g., change in shape orientation, changes in yieldstrength, and/or flexural modulus properties, damping capacity, and thelike. A preferred shape memory alloy is a nickel-titanium based alloycommercially available under the trademark NITINOL from Shape MemoryApplications, Inc.

By heating the material above the reversion temperature of the shapememory alloy contracting constituent forces crack closure. While held atthe reversion temperature, the low melting phase of the matrix partiallyliquifies and acts as a healing agent filling in the crack andsolidifying when brought back to room temperature.

In another embodiment, the contracting constituent comprises materialsthat have a negative temperature coefficient of thermal expansion.Examples of materials that exhibit negative thermal expansion are cubiczirconium tungstate (ZrW₂O₈), members of the AM₂O₈ family of materials(where A=Zr or Hf, M=Mo or W) and ZrV₂O₇. A₂(MO₄)₃ also is an example ofcontrollable negative thermal expansion. These compounds contractcontinuously over a temperature range of 0.3 to 1050 Kelvin.

As the temperature of the composite is increased, the negative expansioncoefficient materials undergo contraction thereby forcing the opposingsides of the crack together, while the low melting phase of the matrixpartially liquifies and acts as a healing agent filling in the crack andsolidifying when brought back to room temperature.

The contracting constituent may be present in an amount of 2 to 40,specifically 3 to 30, and more specifically 4 to 20 weight percent,based on the total weight of the composite.

In one embodiment, in one method of manufacturing an article, theelements used in the metal alloy matrix composite are blended together,and then melted and molded to form the desired article. The elements maybe in pure metal form or may be in the form of alloys, which are thenblended together. When the metal alloy matrix is in the melt (duringproduction), the contracting constituent is introduced into the mixture.It is desirable for the alloy to undergo melting at a temperature thatis below the temperature where the contracting constituent is activated.(e.g., below reversion temperature of the shape memory alloy or belowthe temperature where the material having the negative temperaturecoefficient of expansion is affected).

In one embodiment, the elements used in the metal alloy matrix compositeare blended, melted and/or molded at a temperature of from about 500 toabout 900° C., specifically about 650 to about 850° C., and morespecifically about 700 to about 800° C.

In one embodiment, in one method of using the self-repairing metal alloymatrix composite, the metal alloy matrix composite is heated to apredetermined temperature effective to produce a phase transition in theshape memory alloy element and/or a liquid assisted metal repair of themetal alloy matrix composite due to the partial liquification of the lowmelting eutectic phase. In an embodiment, the predetermined temperatureis at least 500° C.

In one method of using the metal alloy matrix composite, the compositematerial is molded into an article which serves as a component that issubjected to physical stress and/or fatigue during service in forexample, aeronautic applications. Fatigue cracks which are formed duringoperation may be repaired, by heating the component, or the areasurrounding the crack therein, as described above to induce phasetransformation of the shape memory alloy element in the metal alloymatrix composite and/or the partial liquification of the low meltingeutectic phase of the metal alloy matrix composite. In this way, cracksof various degrees, and even cracks which may not be easily detected,may be repaired by the self-repairing metal alloy matrix composite. Themethod of using the self-repairing metal alloy matrix composite may beemployed repeatedly as new cracks propagate through the component duringoperation. The metal alloy matrix composite described herein thusextends the lifetime of the service component.

In one embodiment, following the phase transition of the shape memoryalloy element to close cracks present in the metal alloy matrixcomposite and/or the partial liquification of the low melting eutecticphase to fill in the cracks via liquid assisted metal repair, crackspresent in the metal alloy matrix composite are reduced by 70% orgreater, specifically 80% or greater, more specifically 90% or greater.

After healing, the composites were found to recover over 70%,specifically over 80% and more specifically over 90% of their originaltensile and yield strength.

The metal alloy matrix composite described herein also avoids drawbacksassociated with conventional repair materials and/or techniques sincethe self-repairing metal alloy matrix composite may be utilized torepair cracks therein without the use of additional materials, skilledapplication of the repair technique and/or direct access to thecomposite material or the cracks present therein, and may also be usedto repair components during operation, such as those in space flight,without taking the component or equipment off-line for maintenance andrepairs.

The composite may be employed as an article in aerospace equipment orany other equipment or application for which lightweight materialsand/or improved fatigue performance is desired, including but notlimited to, aeronautical fuel tanks (e.g., in a shuttle or otherspacecraft) and nuclear reactors.

The metal alloy matrix composites, methods of manufacture thereof andarticles comprising the same are exemplified by the followingnon-limiting examples.

EXAMPLES

The example was conducted to demonstrate healing brought about by analuminum (Al)-silicon (Si) system. The Al—Si system was studied becauseit represents an alloy system yielding moderate strength increases, adecrease in eutectic temperature, and the binary alloy known to haveexcellent castability. The phase diagram (see FIG. 2) was calculatedusing Pandat software with the PanMagnesium database. FIG. 2 shows phasediagram of Al—Si below 50% Si. At a composition of Al-3.0 at % Si (3.1wt % Si), a healing temperature of 592° C. will yield 20% liquid. Acomposition of Al-3.0 at % Si (3.1 wt % Si) was selected. At thiscomposition, the healing temperature was calculated to be 592° C. Shouldfluctuations occur within the furnace, 592° C. ±5° C. yields a range ofonly 18-22.6% liquid (see FIG. 3). FIG. 3 shows the phase fraction ofAl-3.0 at % Si at various temperatures. The heat-treatment temperatureof 592° C. reveals a 20% liquid composition during the healing process.The box signifies the potential area of liquid % should the temperaturefluctuate by ±5° C. during heat-treatment.

Al (Al shot, 99.99%, Alfa Aesar) and Si (Si lump, 99.9999%, Alfa Aesar)were melted in a furnace at 850° C. and cast into a graphite mold tocreate an approximately 15% Si master alloy. After verifying thecomposition via ICP, appropriate amounts of the Al—Si master alloy andpure Al were melted at 750° C. until a liquid solution to obtain thedesired Al-3.0 at % Si composition. One NiTi SMA wire, designated BB-35(Ni-49.3 at % Ti, Ø=0.87 mm, Memry Corporation), was laid horizontallyin the graphite tensile bar mold which was heated up to 350° C. beforecasting to prevent casting defects. The Al—Si melt was poured over thewire into the mold and allowed to cool. Each tensile bar was placed intoa furnace for 24 hours at 592° C. and then air-cooled to set theeutectic microstructure (see FIG. 4).

After heat-treatment, the Al—Si tensile composite bars were ground to a320-grit surface finish and tested in tension using the Instron 5582machine at a rate of 1.0%/min. Results of the testing are shown inTable 1. The Al—Si composites showed moderate yield and ultimatestresses and moderate ductility in relation to the other binary alloys.The failures were found to occur along the eutectic regions as shown inFIG. 5. FIG. 5 depicts representative microstructure in an Al-3.0 at %Si (3.1 wt % Si) composite after heat-treatment at 592° C. for 24 hoursshowing failure occurring along eutectic regions.

TABLE 1 0.2% Yield Ultimate V_(f) Wires Modulus Stress Stress FailureSpecimen (%) (GPa) (MPa) (MPa) Strain (%) A 2.26 74.9 38.9 97.2 4.21 B2.00 48.4 40.2 83.3 3.18 C 2.40 63.1 39.0 100.4 4.42 Avg. 2.22 62.1 39.493.6 3.94 ±1σ 0.20 13.3 0.7 9.1 0.66

Following the virgin composite testing, the samples were encapsulatedunder vacuum (in Pyrex lying on a graphite strip and wrapped in Ta foil)and heat-treated a second time for 24 hours at 592° C. Followingair-cooling, the Al—Si composites were tested in tension again forhealing. Using Equation 3.1, the percent healing was calculated for thesamples exhibiting healing (see Table 2).

TABLE 2 ‘Healed’ Ultimate Stress Specimen (MPa) % Heal A 88.1 90.6 B93.7 112.5 C 72.0 71.7 Average 91.6

It was noted that each specimen showed visible signs of healing (seeFIG. 6); i.e., crack size reduction. FIG. 6 shows a comparison of the a)pre-heal to b) post-heal tensile bar in an Al-3.0 at % Si compositereinforced with 2.0 vol % NiTi SMA wire.

A comparison between the virgin and healed tensile behavior of specimen‘A’ is shown in FIG. 7. FIG. 7 shows a comparison of the virgin andhealed tensile behavior of an Al-3.0 at % Si composite reinforced with2.0 vol % NiTi SMA wire. The sample was found to possess 90.6% retainedtensile strength post-healing heat-treatment.

All three of the healed composites were found to retain similar modulusand yield strength values as the virgin composite. For specimen ‘B’, thehigher ultimate strength in the healed composite is attributed tohealing the defect which caused the early failure, thereby allowing fora higher ultimate strength post-healing. In specimen ‘C’, the failure inthe composite post-healing actually occurred in a different place thanthe original crack (see FIG. 8). FIG. 8 shows a comparison of the a)post-tensile 1, b) post-heal and c) post-tensile 2 in an Al-3.0 at % Sicomposite reinforced with 2.4 vol % NiTi SMA wire. The arrow in a) showsthe original composite failure and the healed crack is shown by thearrow in b). In c) the oval shows the second failure that occurredduring second tensile test.

Example 2

This example was conducted to study whether increasing the volumefraction of NiTi wires would increase healing, more Al—Si compositeswere manufactured as above, but with an increased number of NiTi wires.After a heat-treatment of 592° C. for 24 hours, the Al—Si tensilecomposite bars were ground to a 320-grit surface finish and tested intension using the Instron at a rate of 1.0%/min. Results of the testingare shown in Table 3. The new Al—Si composites with nearly double thevolume fraction of SMA wires showed more variance in the resultantelastic modulus, yield and ultimate stresses, and strain to failure inrelation to the lower volume fraction composites. However, the 0.2%yield strength was found to have increased over 20% on average. Theother properties were found to be statistically similar using a T-testwith a 95% confidence interval.

TABLE 3 Mechanical testing results for Al—Si matrices reinforced with3.5-4.5 vol % SMA wire reinforcements V_(f) 0.2% Yield Ultimate WiresModulus Stress Stress Failure Specimen (%) (GPa) (MPa) (MPa) Strain (%)D 4.43 68.7 49.5 108.9 6.24 E 3.79 73.7 45.7 89.9 2.80 Avg. 4.11 71.247.6 99.4 4.52 ±1σ 0.45 3.5 2.7 13.4 2.43

Following the virgin composite testing, the higher volume fractioncomposite bars were encapsulated under vacuum (in Pyrex lying on agraphite strip and wrapped in Ta foil) and heat-treated a second timefor 24 hours at 592° C. Following air-cooling, the Al—Si composites weretested in tension again for healing. The percent healing was calculatedfor the samples exhibiting healing (see Table 4).

TABLE 4 Healing characteristics of Al—Si composites with 3.5-4.5 vol %NiTi wires ‘Healed’ Ultimate Stress Specimen (MPa) % Heal D 35.1 32.2 E50.2 55.8 Average 44.0

It was noted the higher volume fraction Al—Si composites did not heal aswell as the lower volume fraction composites. FIG. 9 shows a comparisonof the virgin tensile behavior of an Al-3.0 at % Si composite reinforcedwith either 2.26 vol % or 4.43 vol % NiTi SMA wire. The higher volumefraction composites were shown to possess greater yield strengths, butexhibited debonding during tensile testing and thus lower retainedstrength values.

Looking at FIG. 9, when comparing specimen A to specimen D, the increaseyield strength is shown. But the tensile data for specimen E also showsnumerous decreases in stress followed by recovery. These decreases wereaccompanied by a cracking sound during testing and are attributed todebonding between the NiTi fibers and Al—Si matrix (see FIG. 10). FIG.10 shows an optical image showing debonding between an Al-3 at % Simatrix and a NiTi wire following tensile testing.

This is thought to be the reason for the lower healing percentages foundin the composites with a greater volume fraction of NiTi. Debondingwould prevent the wires from pulling the matrix together when as thetemperature is increased during a healing heat treatment.

Example 3

Fatigue testing of A1-3Si samples (E466 dogbone geometry) was carriedout in order to determine the stress-life behavior of this alloy.However, there was a large amount of variability in the results, whichcan be attributed to casting defects such as gas evolution and shrinkageporosity due to the casting techniques. Porosity was seen throughout thetested specimen including the gage length, which was analyzed usingmicro CT scanning by collaborators at NASA KSC. These defects providelocations for crack initiation as well as preferred crack propagationpath. Implementation of the previously discussed semisolid processingtechnique is expected to diminish the effects of casting defects andtherefore reduce variability in fatigue life. Scanning electronmicroscopy was conducted to image the fracture surface of a sample afterfatigue failure and it showed linear propagation of the crack until itencounters the SMA reinforcement, which then deflects the crack. Oncenormalized for ultimate tensile strength (UTS), Al-3Si show similarfatigue life trends when compared with a conventionally cast A201 Alalloy, as shown in FIG. 11. FIG. 11 depicts a comparison of S—N datafrom Al-3Si and A201, normalized with respect to UTS. This is anindication that the Al3-Si has the ability to rival conventional alloys.

An additional issue encountered with fatigue testing of conventionaldogbone Al—Si samples was that stress-controlled fatigue testing createdoverload failure at the conclusion of the test, which proved toocatastrophic to heal using this MMC system. In order to better monitorand control crack size and growth, fatigue crack growth tests wereconducted on middle tension (M(T)) and single edge notch tension(ESE(T)) specimen to produce and heal a small fatigue crack. The M(T)samples were machined from a larger casting of the Al—Si alloy withembedded SMA wires with geometry based on ASTM Standard E647. Eachsample had one embedded wire on either side of the machined notch, andtesting was carried out with a stress amplitude ratio of R=0.1, maximumstress at 0.8 of the yield strength, and 80 Hz frequency. Crack growthmeasurements were taken from the center of the specimen, via interruptedtesting and optical microscopy.

It was observed that cracking initiates from the notch and propagatesmostly intergranularly, following the eutectic regions. After crackpropagation, samples were heat treated to heal the fatigue cracking,optical microscopy of the healed crack region was conducted, and thensamples were re-tested in fatigue. The cracking path in one such M(T)sample can be seen in FIG. 12, along with the healed crack. Mostnotably, the eutectic distribution has been modified by heat treatment.FIG. 12 shows optical micrographs of cracks extending from apre-machined notch in M(T) sample. Healing of the sample lead to crackclosure as well as altered crack path upon further fatigue cracking.Although the previous crack may have led to a faster crack initiationand the presence of eutectic near the machined crack still provides apreferred crack path, the local modification of the microstructureproved to be beneficial. Due to the eutectic redistribution near thecrack tip, the crack path was altered during subsequent fatigue testing.This secondary crack is characterized by a larger amount of crackdeflection, which results in an overall lower crack growth rate, assummarized in FIG. 11. This also indicates that fatigue crack growthrate is highly dependent on local microstructure near the crackinitiation site and discontinuous eutectic would improve fatigue life.It should be noted that the first recorded crack length was atapproximately 100k cycles, where the first measureable fatigue crack wasseen, and does not correspond to the machined notch length. Variance inpost-heal crack length can be attributed to differences in amount ofcrack healing, as well as the amount of localized eutecticredistribution during healing.

FIG. 13 shows fatigue crack growth rate from M(T) testing of Al—Si SMASHalloy. All cracks showed some crack closure, as well as reduction incrack growth rate. The grey area represents the location of SMAreinforcement.

In addition to efforts made at the UF on fatigue testing M(T) samples,ESE(T) samples were also tested in fatigue at Langley Research Center(LaRC). Specimens were made according to ASTM standard E647, testedunder a constant K control, and consisted of Al—Si matrix material. TheESE(T) samples showed cracking similar to M(T) samples; cracking wasobserved to occur from the notch, propagating primarily intergranularlythrough the matrix. After sample fatigue cracking, the materialsunderwent the healing heat treatment under vacuum. Microstructuralanalysis was used to determine healing efficiency. FIG. 14 shows how thecrack propagated after healing. While the crack initially propagatesintergranularly following the eutectic along the grain boundaries, itchanges to transgranular propagation once the crack extended beyondpreviously healed sections of the matrix material. FIG. 14 shows opticalMicrograph of AlSi ESE(T) sample after secondary fatigue cracking postinitial healing. Healing treatment led to crack closure and aftercontinued testing, resulted in an altered crack path changing fromintergranular propagation along the eutectic regions to transgranularpropagation through the matrix (point “A”). This behavior is seen due tothe liquid-assisted nature of the healing process, in which the liquidusphase is supplied via preferential melting of the eutectic. This liquidphase flows to the cracked region and solidifies there upon cooling. Aswith the M(T) samples, fatigue crack growth rates were highly dependenton the local microstructure.

After microstructural analysis, one ESE(T) was subsequently re-tested infatigue to obtain an understanding of multiple healing/fatigue cyclebehavior. The sample was again healed under a heat treatment, and thenagain fatigued. In total, this sample received 2 healing cycles, and 3fatigue cycles. FIG. 15 below shows a graph of crack length versus thenumber of cycles for all three fatigue tests. FIG. 15 shows ESE(T) crackgrowth rate of Al—Si matrix after two healing treatment cycles. Allcracks showed some crack closure despite the lack of SMA wires, inaddition to a reduction in crack length.

Changes in crack growth behavior were observed to take place after eachrepeated healing, indicating a transition from intergranular totransgranular fracture mechanism. It can also be noted that each healingevent heals less material than previous healing events; this ispotentially due to the absence of SMA wires, which provide pull-backforce in the composite. This force allows for a more complete reductionin crack length. Fatigue cracks usually propagate transgranularly and itis likely that the liquid-assisted healing treatment creates a networkof eutectic phase within the microstructure that affects crackpropagation. It was shown that cracks propagate intergranularly alongthe eutectic network through healed regions and then transgranularlythrough material that had not been damaged previously. It is anindication that the healing treatment will close existing fatigue cracksand may deter future crack propagation. However, the healed regions arenot as effective as virgin matrix regions in crack retardation. Thisdecrease in fatigue damage tolerance may be alleviated by a secondaryheat treatment after the initial healing treatment.

While this disclosure describes exemplary embodiments, it will beunderstood by those skilled in the art that various changes can be madeand equivalents can be substituted for elements thereof withoutdeparting from the scope of the disclosed embodiments. In addition, manymodifications can be made to adapt a particular situation or material tothe teachings of this disclosure without departing from the essentialscope thereof. Therefore, it is intended that this disclosure not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this disclosure.

What is claimed is:
 1. A composite comprising: a metal alloy matrix;where the metal alloy matrix comprises aluminum in an amount greaterthan 50 atomic percent; a first metal and a second metal; where thefirst metal is different from the second metal; and where the metalalloy matrix comprises a low temperature melting phase and a hightemperature melting phase; where the low temperature melting phase meltsat a temperature that is lower than the high temperature melting phase;and a contracting constituent; where the contracting constituent exertsa compressive force on the metal alloy matrix at a temperature between amelting point of the low temperature melting phase and a melting pointof the high temperature melting phase or below the temperature of thelow temperature melting phase.
 2. The composite of claim 1, where thefirst metal is copper and where the second metal is silicon.
 3. Thecomposite of claim 1, where the aluminum is present in an amount of 65to 96 atomic percent, based on a total atomic composition of the metalalloy matrix.
 4. The composite of claim 2, where the copper is presentin an amount of 2 to 18 atomic percent, based on a total atomiccomposition of the metal alloy matrix.
 5. The composite of claim 2,where the silicon is present in an amount of 1 to 17 atomic percent,based on a total atomic composition of the metal alloy matrix.
 6. Thecomposite of claim 1, where the contracting constituent is a shapememory alloy.
 7. The composite of claim 6, where the shape memory alloyis selected from Ti(NiPt), Ti(NiPd), Ti(NiAu), (TiHf)Ni, NiTi, NiTiSn,indium-titanium based alloys, nickel-aluminum based alloys,nickel-gallium based alloys, copper based alloys, gold-cadmium basedalloys, silver-cadmium based alloys, indium-cadmium based alloys,manganese-copper based alloys, iron based alloys, and a combinationthereof.
 8. The composite of claim 1, where the contracting constituentcomprises a material having a negative coefficient of thermal expansion.9. The composite of claim 8, where the material having a negativecoefficient of thermal expansion is selected from the group consistingof ZrW₂O₈, ZrMo₂O₈, HfW₂O₈, HfMo₂O₈ and ZrV₂O₇.
 10. The composite ofclaim 1, wherein cracks present in the composite are reduced by 80% orgreater.
 11. The composite of claim 1, wherein the melting point of thelow temperature melting phase and the melting point of the hightemperature melting phase are at least 50° C. apart from each other. 12.An article comprising the composition of claim
 1. 13. The article ofclaim 12, wherein the article is a component which has been in operationin aerospace equipment.
 14. A method comprising: blending aluminum, afirst metal, a second metal and a contracting constituent to form acomposite; where the first metal is different from the second metal; andwhere the aluminum, the first metal and the second metal form a metalalloy matrix that comprises a low temperature melting phase and a hightemperature melting phase; where the low temperature melting phase meltsat a temperature that is lower than the high temperature melting phase;where the contracting constituent exerts a compressive force on themetal alloy matrix at a temperature between a melting point of the lowtemperature melting phase and a melting point of the high temperaturemelting phase or at a temperature below the low temperature meltingphase; and forming the composite into an article.
 15. The method ofclaim 14, where the first metal is copper and where the second metal issilicon.
 16. The method of claim 14, where the aluminum is present in anamount of 50 to 96 atomic percent, based on a total atomic compositionof the metal alloy matrix.
 17. The method of claim 15, where the copperis present in an amount of 2 to 18 atomic percent, based on a totalatomic composition of the metal alloy matrix.
 18. The method of claim15, where the silicon is present in an amount of 1 to 17 atomic percent,based on a total atomic composition of the metal alloy matrix.
 19. Themethod of claim 1, where the contracting constituent is a shape memoryalloy.
 20. A method comprising: heating a composite comprising: a metalalloy matrix; where the metal alloy matrix comprises aluminum in anamount greater than 50 atomic percent; a first metal and a second metal;where the first metal is different from the second metal; and where themetal alloy matrix comprises a low temperature melting phase and a hightemperature melting phase; where the low temperature melting phase meltsat a temperature that is lower than the high temperature melting phase;and a contracting constituent; where the contracting constituent exertsa compressive force on the metal alloy matrix at a temperature between amelting point of the low temperature melting phase and a melting pointof the high temperature melting phase; and cooling the composite.
 21. Acomposite comprising: a metal alloy matrix; where the metal alloy matrixcomprises aluminum in an amount greater than 50 to 99 atomic percent; afirst metal dispersed in the metal alloy matrix, where the first metalis present in an amount of 1 to 50 atomic percent; and where the metalalloy matrix comprises a low temperature melting phase and a hightemperature melting phase; where the low temperature melting phase meltsat a temperature that is lower than the high temperature melting phase;and a contracting constituent; where the contracting constituent exertsa compressive force on the metal alloy matrix at a temperature between amelting point of the low temperature melting phase and a melting pointof the high temperature melting phase.
 22. The composite of claim 21,where the first metal is silicon.
 23. The composite of claim 21, wherethe aluminum is present in an amount of 80 to 98 atomic percent, basedon a total atomic composition of the metal alloy matrix.
 24. Thecomposite of claim 22, where the silicon is present in an amount of 2 to18 atomic percent, based on a total atomic composition of the metalalloy matrix.
 25. The composite of claim 21, where the contractingconstituent is a shape memory alloy.
 26. The composite of claim 25,where the shape memory alloy is selected from Ti(NiPt), Ti(NiPd),Ti(NiAu), (TiHf)Ni, NiTi, NiTiSn, indium-titanium based alloys,nickel-aluminum based alloys, nickel-gallium based alloys, copper basedalloys, gold-cadmium based alloys, silver-cadmium based alloys,indium-cadmium based alloys, manganese-copper based alloys, iron basedalloys, and a combination thereof.
 27. The composite of claim 21, wherethe contracting constituent comprises a material having a negativecoefficient of thermal expansion.
 28. The composite of claim 27, wherethe material having a negative coefficient of thermal expansion isselected from the group consisting of ZrW₂O₈, ZrMo₂O₈, HfW₂O₈, HfMo₂O₈and ZrV₂O₇.
 29. The composite of claim 21, wherein cracks present in thecomposite are reduced by 80% or greater.
 30. The composite of claim 21,wherein the melting point of the low temperature melting phase and themelting point of the high temperature melting phase are at least 50° C.apart from each other.
 31. An article comprising the composition ofclaim
 21. 32. The article of claim 31, wherein the article is acomponent which has been in operation in aerospace equipment.
 33. Amethod comprising: blending aluminum a first metal, and a contractingconstituent to form a composite; and where the aluminum and the firstmetal form a metal alloy matrix that comprises a low temperature meltingphase and a high temperature melting phase; where the low temperaturemelting phase melts at a temperature that is lower than the hightemperature melting phase; where the contracting constituent exerts acompressive force on the metal alloy matrix at a temperature between amelting point of the low temperature melting phase and a melting pointof the high temperature melting phase; and forming the composite into anarticle.
 34. The method of claim 33, where the first metal is silicon.35. A method comprising: heating a composite comprising: a metal alloymatrix; where the metal alloy matrix comprises aluminum in an amountgreater than 50 to 99.9 atomic percent; a first metal dispersed in themetal alloy matrix, where the first metal is present in an amount of 0.1to 50 atomic percent; and where the metal alloy matrix comprises a lowtemperature melting phase and a high temperature melting phase; wherethe low temperature melting phase melts at a temperature that is lowerthan the high temperature melting phase; and a contracting constituent;where the contracting constituent exerts a compressive force on themetal alloy matrix at a temperature between a melting point of the lowtemperature melting phase and a melting point of the high temperaturemelting phase or below the melting point of the low temperature meltingphase; and cooling the composite.